Hello all ! I am trying to evaluate the electrical power consumption of a 3 phase , 1 hp, wye connected pump motor for a piece of equipment here at work. The motor literature says that it has a power factor of 71. At the electrical control box , I measure 208 vac across each phase going out to the motor, and with a clamp on amp probe, I measure about 2 amps on each of the 3 phases. To calculate electrical power consumption, do I simply take my measurements and multiply V x I for each wire and add up the three results, or is there more to it than that? Thanks!

 

There's more to it.

You need a wattmeter that can measure power on a 3 phase system, like this old electromechanical meter:

http://www.ebay.com/itm/WESTON-POLY...064?pt=LH_DefaultDomain_0&hash=item2573a47a88

There are modern electronic versions available.

 

certainly using wattmeter is the easiest way. if you want to calculate power then:

Pdc=V*I
Pac1=V*I*cos(θ)
Pac3=sqrt(3)*VLL*I*cos(θ)
where cos(θ) is phase difference between V and I. you may use value stamped on the motor. note it is 0.71, not 71.

If you measured voltages phase to neutral (Y circuit) then
Pac3=3*VLN*I*cos(θ)

because VLL = VLN*sqrt(3) in a 3-phase circuit
VLL= Voltage Line to Line (phase to phase)
VLN = Voltage Line to Neutral

 

Thanks for all your help. Since this is just for a ballpark estimate and don't have a watt meter at my disposal, I'll just go with calculations based on my readings.

 

3phase power calculations

Total power drawn from supply W = V x I x 1.732

actual load power given out W = V x I x 1.732 x PF(power factor)

so in your case 208V x 2Amps x 1.732= 720.512W actual power drawn from supply.
Motor power given out is 720.512 x 0.71 = 511.56W

so the higher to a power factor of 1 is better as your wasting money!!!